1. Field of the Invention
The present invention relates to a method of fabricating a flash memory. More particularly, the present invention relates to a method for fabricating a flash memory having a bipolar junction transistor used to do an erase operation.
2. Description of the Related Art
The basic operations of a memory device are write data and read data. The write operation of a flash memory includes a program operation and an erase operation. These two operations are performed by adjusting the threshold voltage of a floating gate. The data stored in the flash memory is changed by performing the program operation and the erase operation.
Fowler-Nordheim tunneling theory is used to perform the erase operation in a p-channel flash memory. Channel hot carrier injection theory or Band-to-Band tunneling theory is used to perform the program operation.
The drawbacks of using Fowler-Nordheim tunneling for the erase operation are low operation efficiency, poor endurance capability, etc.
FIG. 1 is a schematic, cross-sectional diagram used to depict the erase operation of a conventional flash memory.
Referring to FIG. 1, a control gate voltage (V.sub.eg) is much larger than 0V while performing the erase operation. A drain voltage (V.sub.d) and a source voltage (V.sub.g) are floating, thus no hot electrons flow between a source region and a drain region. The substrate voltage (V.sub.sub) is 0V or negative bias. Electrons in the channel tunnel to a floating gate because of Fowler-Nordheim tunneling.
In the method mentioned above, the electrons in the channel tunnel to the floating gate because of a high electrical field produced by the control gate voltage (V.sub.eg) and the substrate voltage (V.sub.sub). A tunneling oxide layer between the substrate and the floating gate suffers the high electrical field, so that the lifetime and reliability of the tunneling oxide layer are affected.